The invention relates to a process for producing a steel strip or sheet, in which liquid steel is cast in a continuous-casting machine to form a thin plate and, while making use of the casting heat, is fed through a furnace device, is roughed in a roughing stand to a pass-over thickness and is rerolled in a finishing rolling stand to form a steel strip or sheet of the desired final thickness, and to a device which is suitable for use in such a process.
Where the following text refers to a steel strip, this is also to be understood as including a steel sheet. A thin plate is understood to mean a plate whose thickness is less than 150 mm, preferably less than 100 mm.
A process of this kind is known from European Patent Application 0 666 122.
This Patent Application describes a process in which a continuously cast thin steel plate, after being homogenized in a tunnel furnace device, is rolled in a number of hot-rolling steps, i.e. in the austenitic field, to form a strip having a thickness of less than 2 mm.
In order to achieve such a final thickness using rolling devices and rolling trains which can be realized in practice, it is proposed to reheat the steel strip, preferably by means of an induction furnace, at least after the first rolling mill stand.
A separating device is positioned between the continuous-casting machine and the tunnel furnace device, which device is used to cut the continuously cast thin plate into pieces of approximately equal length, which pieces are homogenized in the tunnel furnace device at a temperature of approx. 1050xc2x0 C. to approx. 1150xc2x0 C. After leaving the tunnel furnace device, the pieces may if desired be cut again into half-plates which have a weight which corresponds to the coil weight of the wound coil to which the steel strip is wound downstream of the rolling device.
The object of the invention is to provide a process of the known type which offers more options and with which, moreover, steel strip or sheet can be produced in a more efficient way. To this end, the process according to the invention is characterized in that
a. to produce a ferritically rolled steel strip, the strip, the plate or a part thereof is fed without interruption at least from the furnace device, at speeds which essentially correspond to the speed of entry into the roughing stand and the following reductions in thickness, from the roughing stand to a processing device which is disposed downstream of the finishing rolling stand, the strip coming out of the roughing stand being cooled to a temperature at which the steel has an essentially ferritic structure;
b. to produce an austenitically rolled steel strip, the strip coming out of the roughing roll is brought to or held at a temperature in the austenitic range, and in the finishing rolling stand it is rolled to the final thickness essentially in the austenitic field and is then cooled, after this rolling, to the ferritic field.
In this context, strip is understood to mean a plate of reduced thickness.
In the conventional method for producing ferritic, or cold-rolled, steel strip, the starting point is a hot-rolled roll of steel, as is also produced using the known method from EP 0,666,112. A hot-rolled roll of steel of this kind usually has a weight of between 16 and 30 tonnes. In this case, the problem arises that it is very difficult, with large width/thickness ratios of the steel strip obtained, to control the dimensions of the strip, i.e. the thickness profile over the width of the strip and over the length of the strip. Owing to the discontinuity in the stream of material, the top and tail of the hot-rolled strip behave differently from the central part in the rolling device. Controlling the dimensions represents a problem above all during entry and exit of the hot-rolled strip into and out of the finishing rolling stand for ferritic or cold rolling. In practice, advanced forwards and self-adapting control systems and numerical models have been used in an attempt to keep the top and tail, which have incorrect dimensions, as short as possible. Nevertheless, every roll has a top and tail which is to be rejected and may amount to up to a few tens of meters in length.
In the installations currently used, a width/thickness ratio of about 1200-1400 is regarded as the maximum which can be achieved in practice: a grater width/thickness ratio leads to an excessively long top and tail before reaching a stable situation, and hence to excessive levels of scrap.
On the other hand, with a view to efficiency of materials when working a hot-rolled or cold-rolled steel strip, there is a need for a greater width with an identical or reduced thickness. Width/thickness ratios of 2000 or more are desired in the market, but cannot be achieved in practice with the known process, for the reasons described above.
The process according to the invention makes it possible to rough the steel strip, at any rate from the furnace device, in an uninterrupted or continuous process in the austenitic field, to cool it to the ferritic field and to roll it in the ferritic field to give the final thickness.
A much simpler feedback control has proven sufficient for controlling the dimensions of the strip.
The invention also makes use of the insight that it is possible to employ the process with which, according to the prior art, only hot-rolled steel strip is produced, in such a manner, while making use of essentially the same means, that this process can also be used to obtain, in addition to an austenitically rolled steel strip, a ferritically rolled steel strip as well, having the properties of a cold-rolled steel strip.
This opens up the possibility of using a device which is known per se to produce a wider range of steel strips, and more particularly to produce steel strips which have a considerably higher added value on the market. In addition, the process yields a particular advantage when rolling a ferritic strip according to step a, as will be explained in the following text.
The invention also makes it possible to achieve a number of other important advantages, as will be described in the following text.
When carrying out the process according to the invention, it is preferred for the roughing to take place in the austenitic field, as soon as possible downstream of the furnace device in which the plate is homogenized at temperature. Furthermore, it is preferred to select a high rolling speed and reduction. In order to obtain constant properties for the steel, it is necessary to prevent the plate, or at least an excessive part thereof, from passing into the two-phase field in which the austenitic and ferritic structures exist next to one another. After leaving the furnace device, the homogenized austenitic plate cools most rapidly at the side edges. It has been found that cooling takes place primarily over an edge part of the plate which has a width which is comparable to the current thickness of the plate or strip. By rolling the strip shortly after it leaves the furnace, and preferably with a considerable reduction, the extent of the cooled edge part is limited. It is then possible to produce a strip of the correct strip shape and with constant, predictable properties over virtually the entire width.
The virtually homogeneous temperature distribution over the width, together with the thickness of the plate, provides the additional advantage of a broader working range within which the invention can be employed. Since it is undesirable to carry out rolling in the two-phase field, the working range with regard to the temperature is limited on the underside by the temperature of that part of the plate which first passes into the two-phase field, i.e. the edge region. In the conventional process, the temperatures of the central part is then still fat above the transition temperature at which austenite begins to change into ferrite. In order nevertheless to be able to exploit the higher temperature of the central part, it is proposed in the prior art to reheat the edges. Using the invention, this measure is not necessary, or at least is necessary to a considerably reduced extent, and the result is that the austenitic rolling process can be continued until virtually the entire plate, in particular in the width direction, is at a temperature close to the transition temperature.
The more uniform temperature distribution prevents the situation where a relatively small part of the plate has already passed into the two-phase field, thus making further rolling undesirable, while a large part is still well in the austenitic field and thus could still be rolled. It should also be considered here that on cooling from the austenitic field over a relatively small temperature span of the temperature range within which transformation occurs, a large proportion of the material is transformed. This means that even a small fall below the transition temperature results in a large part of the steel being transformed. For this reason, in practice there is considerable anxiety about falling below the highest temperature of this temperature range.
More detailed embodiments of the invention and a device for carrying out the invention, as well as exemplary embodiments, are described in Patent Application NL-1003293, which is hereby deemed to be incorporated in its entirety in this patent.
The invention is particularly suitable for use in the production of deep-drawing steel. In order to be suitable as a deep-drawing steel, a steel grade has to satisfy a number of requirements, of which a few important ones are discussed below.
To obtain a closed so-called two-part can, the first part of which comprises the base and the body and the second part of which is the lid, the basis for the first part is a planar blank made of deep-drawing steel, which is first deep-drawn to form a cup having a diameter of, for example, 90 mm and a height of, for example, 30 mm, the walls of which cup are then drawn to form the can having a diameter of, for example, 66 mm and a height of, for example, 115 mm. Indicative values for the thickness of the steel material in the various production phases are: initial thickness of the blank 0.26 mm, base thickness and wall thickness of the cup 0.26 mm, base thickness of the can 0.26 mm, wall thickness of the can half-way up 0.09 mm, thickness of the top edge of the can 0.15 mm.
Deep-drawing steel has to be extremely ductile and remain so over the course of time, i.e. it must not age. Ageing leads to high deformation forces, crack formation during the deformation and surface defects owing to flow lines. One way of counteracting the ageing is the so-called overageing by precipitation of carbon.
The desire to save material by being able to make ever lighter cans also has an effect on the requirement of high ductility in order, starting from a given initial thickness of the blank, to be able to achieve a minimum possible final thickness of the can wall and also of the top edge of the can. The top edge of the can places particular demands on the deep-drawing steel. After forming the can by drawing the walls, the diameter of the top edge is reduced, by the process known as necking, in order to be able to use a smaller lid, thus saving on lid material. After the necking, a flange is provided along the top of the top edge in order to be able to attach the lid. The necking and the provision of the flange, in particular, are processes which place high demands on the additional ductility of the deep-drawing steel, which had previously already been deformed during the fabrication of the body.
In addition to the ductility, the purity of the steel is important. Purity is in this case understood to mean the extent to which inclusions, mostly oxidic or gaseous inclusions, are absent. Inclusions of this kind are formed when making steel in an oxygen steel-making plant and from the casting powder which is used in the continuous casting of the steel plate which forms the starting material for the deep-drawing steel. During necking or forming of the flange, an inclusion may lead to a crack, which itself is in turn the cause of a subsequent leak in the can which has been filled with its contents an then closed. During storage and transportation, contents leaking out of the can may, as a result of contamination in particular, cause damage to other cans and goods around it which amounts to many times the value of just the leaking can with its contents. As the thickness of the edge of the can decreases, the risk of a crack resulting from an inclusion increases. Therefore, deep-drawing steel should be free of inclusions. Insofar as inclusions are inevitable in the current method of steel making, their dimensions are to be kept as small as possible, and they should only occur in very small numbers.
Yet a further requirement relates to the level of anisotropy of the deep-drawing steel. When producing a deep-drawn/wall-drawn or wall-thinned two-part can, the top edge of the can does not run in a planar surface, but rather has a wave pattern around the circumference of the can. In specialist circles, the wave crests are referred to as ears. The tendency to earing is a result of anisotropy in the deep-drawing steel. The ears have to be cut down to the level of the lowest trough, in order to obtain a top edge which runs in a flat surface and can be deformed into a flange, and this process leads to a loss of material. The level of earing is dependent on the total cold-rolling reduction and on the carbon concentration.
It is usual, for considerations of process engineering, to start from a hot-rolled sheet or strip having a thickness of 1.8 mm or more. With a reduction of about 85%, this leads to a final thickness of approx. 0.27 mm. In view of the desire to minimize the consumption of material for each can, a lower final thickness, preferably of lower than 0.21 mm, is desired. Guideline values of approx. 0.17 mm are already being mentioned. At a given starting thickness of approx. 1.8 mm, this therefore requires a reduction of more than 90%. With the usual carbon concentrations, this leads to severe earing, and thus, as a result of these ears being cut off, to additional loss of material, thus negating part of the benefit gained from a lower thickness. A solution has been sought in the use of extra-low or ultra-low carbon steel (ULC steel). Steel of this kind, which has generally accepted carbon concentrations of below 0.01%, down to values of 0.001% or lower, is made by blowing more oxygen into the steel melt in an oxygen steel-making plant, so that more carbon is burnt. If desired, this may be followed by a vacuum pan treatment, in order to reduce the carbon concentration further. As a result of introducing more oxygen into the steel melt, this also results in undesirable metal oxides in the steel melt, which remain as inclusions in the cast steel plate, and later in the cold-rolled strip. The effect of inclusions is magnified by the lower final thickness of the cold-rolled steel. As has been discussed, inclusions are damaging, since they can lead to crack formation. As a result of the lower final thickness, this damaging effect applies a fortiori to ULC steel. The result is that the yield of ULC steel grades for packaging purposes is low, owing to the high level of scrap.
Another object of the invention is to provide a process for producing a deep-drawing steel from steel grades of the low-carbon steel class, which is usually understood to mean a carbon content of between 0.1% and 0.01%, making it possible to achieve a low final thickness with a high yield of the material and also allowing other advantages to be achieved. According to the invention, this method is characterized in that the steel strip is a low-carbon steel having a carbon content of between 0.1% and 0.01% and is cooled, at a pass-over thickness of less than 1.8 mm, from the austenitic field to the ferritic field, and the total reduction by rolling in the ferritic field is less than 90%. The level of anisotropy is dependent on the carbon concentration and the total rolling reduction to which the deep-drawing steel has been subjected in the ferritic field.
The invention is based on the further insight that the total reduction in the ferritic field after transition from the austenitic field is important for the earing, and that earing can be prevented or limited, when cold-rolling in the ferritic field, by keeping the reduction within a defined limit, for a given carbon content, by entering the ferritic field with a sufficiently thin strip.
A preferred embodiment of the process according to the invention is characterized in that the total reduction brought about by rolling in the ferritic field is less than 87%. The level of rolling reduction at which minimum anisotropy occurs is dependent on the carbon concentration and increases as the carbon concentration falls. For low-carbon steel, the cold-rolling reduction which produces minimum anisotropy and hence minimum earing, lies in the range of less than 87%, or more preferably less than 85%. In conjunction with good deformation properties, it is preferred for the total reduction to be more than 75%, and more preferably more than 80%.
The reduction to be carried out in the ferritic field can be kept low, at a low end thickness, in another embodiment of the invention which is characterized in that the pass-over thickness is less than 1.5 mm.
The process indicated provides a deep-drawing steel which can be produced in the known manner using a generally known device and which makes it possible to produce thinner deep-drawing steel than was hitherto possible. Known techniques can be used for rolling and further processing in the ferritic field.